Image projection is a technique commonly used nowadays. However, there exists some deformation between original images and projected images generated by projecting the original images. Therefore, image calibration solutions must be provided in the image projection apparatuses to compensate the deformation generated from projecting the images. For example, keystone adjustment which adjusts a projecting angle from the image projection apparatus is provided to calibrate a trapezoid projected image.
In the application of HUD (Head Up Display), the original image is projected and the projected image is formed through using a front cover of safety helmet or a windshield, which is usually a curved surface. However, the projected image would be twistedly deformed because the curved surface is used for forming the projected image, and the twisted deformation generated due to using the curved surface cannot be calibrated by simply applying the keystone adjustment mentioned above. A most common solution for calibrating the twisted deformation is to design a specific optical element group or to adjust parameters of the optical elements. However, once the curvature of the curved surface is changed, a great effort must be done for redesigning the specific optical element group or readjusting the parameters of the optical elements so as to again compensate the deformation. Therefore, there's a need to establish a solution which can easily calibrate the deformation occurred while projecting images.